1. Field of the Invention
The present invention relates to an image display device, image display method to be used for the image display device, and more particularly to the image display device suitably employed when each pixel of a display panel such as a liquid crystal panel made up of sub-pixels of three primary colors that produce a white color by additive mixture and a sub-pixel of a white color, the image display method to be used for the image display device, such as a liquid crystal display device.
2. Description of the Related Art
In an image display device such as a liquid crystal television set, three primary colors of red (R), green (G), and blue (B) are generally used as basic colors and a color image is displayed by controlling a gray level of each of these three primary colors and by mixing these colors. The image display device as described above obtains white by outputting a specified amount of each of the R, G, and B and performing the additive mixture of colors. However, luminance of the white color produced by the color mixture of these three colors lowers due to its passage through a color layer making up a color filter. In order to mitigate this problem, a related image display device has been proposed which is so configured that its luminance is improved by making up each pixel of its display panel using sub-pixels of R, G, and B that are made to pass through the color layer and a sub-pixel of white (W) that is not made to pass through the color layer.
As related technology of this type, a display device is disclosed in Patent Reference 1 [Japanese Patent Application Laid-open No. 2004-295086 (Page 11 and FIG. 12)] in which, when it is presumed that each of R, G, and B making up an RGB video signal is made up of 8 bit data width (gray level values are 0, 1, . . . , 254, 255), that each of the gray level values is, for example, 255 for R, 217 for G, and 186 for B, and that each gamma (γ) corresponding to the display panel is 2.2; as shown in FIG. 13, in a gamma conversion processing (Step A1), each gray level value of R, G, and B making up the RGB video signal is converted respectively into each of luminance values R0, G0, and B0 (0 to 1) by the following equation:R0=[R/(total gray level number−1)]γ=[255/(256−1)]2.2=1G0=[G/(total gray level number−1)]γ=[217/(256−1)]2.2≈0.7B0=[B/(total gray level number−1)]γ=[186/(256−1)]2.2≈0.5  Equation (1)
In a position coordinate calculating processing (Step A2), position coordinates of an RGB video signal in a Gamut region are calculated and the maximum luminance value M1 and minimum luminance value M2 of the luminance values R0, G0, and B0 obtained by the equation (1) are calculated by the following equation (2):M1=Max(R0,G0,B0)=1M2=Min(R0,G0,B0)=0.5  Equation (2)
In a scaling calculation processing (Step A3), to which portion of the Gamut region an RGB video signal belongs is judged and a scaling value S1 required to convert the maximum luminance value M1 and minimum luminance value M2 into a luminance value of each of R, G, and B in a region where the luminance has been expanded by the addition of a white sub-pixel is calculated by the following equation (3):Region selection: M1−2×M2=1−2×0.5=0Judging condition: since the value is not >0, the routine proceeds to “NO” step and, therefore, S1=2  Equation (3)
In an RGB increased value calculating processing (Step A4), a luminance value of each of the R, G, and B in the region where the luminance has been expanded is calculated from the luminance values R0, G0, and B0 and the scaling value S1 is obtained by the following equation (4):R2=S1×R0=2×1=2G2=S1×G0=2×0.7=1.4B2=S1×B0=2×0.5=1  Equation (4)
In a white color signal extraction processing (Step A5), a white color signal is extracted from the luminance values R2, G2, and B2 and a white color luminance value Wout is calculated by the following equation (5):Wout=Min(R2,G2,B2)=1  Equation (5)
In an RGB signal determining processing (Step A6), the luminance value of each of R, G, B and W is calculated from the luminance values R2, G2, and B2 and white color luminance value Wout in a manner in which each of the luminance values is converted to be 1 or less by the following equation (6):Wout=1Rout=R2−Wout=2−1=1Gout=G2−Wout=1.4−1=0.4Bout=B2−Wout=1−1=0  Equation (6)
In a reverse gamma conversion processing (Step A7), as shown in the following equation (7), the luminance values Rout, Gout, Bout and Wout are converted respectively into gray level values Rout′, Gout′, Bout′ and Wout′ (0, 1, . . . , 254, 255).
                                                                                          R                  out                  ′                                =                                                      (                                                                  total                        ⁢                                                                                                  ⁢                        gray                        ⁢                                                                                                  ⁢                        level                        ⁢                                                                                                  ⁢                        number                                            -                      1                                        )                                    ×                                      R                    out                                          (                                              1                        /                        γ                                            )                                                                                                                                              =                                                                            (                                              256                        -                        1                                            )                                        ×                                          1                                              (                                                  1                          /                          2.2                                                )                                                                              =                  255                                                                    ⁢                                  ⁢                                                                              G                  out                  ′                                =                                ⁢                                                      (                                                                  total                        ⁢                                                                                                  ⁢                        gray                        ⁢                                                                                                  ⁢                        level                        ⁢                                                                                                  ⁢                        number                                            -                      1                                        )                                    ×                                      G                    out                                          (                                              1                        /                        γ                                            )                                                                                                                                              =                                ⁢                                                                            (                                              256                        -                        1                                            )                                        ×                                          0.4                                              (                                                  1                          /                          2.2                                                )                                                                              ≈                  168                                                                    ⁢                                  ⁢                                                                              B                  out                  ′                                =                                                      (                                                                  total                        ⁢                                                                                                  ⁢                        gray                        ⁢                                                                                                  ⁢                        level                        ⁢                                                                                                  ⁢                        number                                            -                      1                                        )                                    ×                                      B                    out                                          (                                              1                        /                        γ                                            )                                                                                                                                              =                                                                            (                                              256                        -                        1                                            )                                        ×                                          0                                              (                                                  1                          /                          2.2                                                )                                                                              =                  0                                                                    ⁢                                  ⁢                                                                              W                  out                  ′                                =                                ⁢                                                      (                                                                  total                        ⁢                                                                                                  ⁢                        gray                        ⁢                                                                                                  ⁢                        level                        ⁢                                                                                                  ⁢                        number                                            -                      1                                        )                                    ×                                      W                    out                                          (                                              1                        /                        γ                                            )                                                                                                                                              =                                ⁢                                                                            (                                              256                        -                        1                                            )                                        ×                                          1                                              (                                                  1                          /                          2.2                                                )                                                                              =                  255                                                                                        Equation        ⁢                                  ⁢                  (          7          )                    
By the above processing, the RGBW video signal is generated from the RGB video signal.
Moreover, a liquid crystal display device described in Patent Reference 2 [Japanese Patent Application Laid-open No. 2006-317898 (Page 2, FIGS. 1 and 2) is provided with a liquid crystal panel having sub-pixels of four colors. A plurality of pieces of white data is extracted by a data converting section by using three source data inputted from outside and any one of a plurality of pieces of the white data extracted by a selecting signal fed from the outside is selected and three pieces of the source data are converted into four pieces of data. Four color data fed from the data converting section is supplied by a timing controller to a data driver and a gate driver and the data driver are controlled and a scan pulse is supplied by the data driver to each of the above sub-pixels and, at the same time, a video data signal is fed to each of the above sub-pixels.
In the data converting section, a reverse gamma correction is made by a reverse gamma correcting section to three color source data to generate three color corrected data and the maximum and minimum luminance values are detected by a luminance detecting section using the three color corrected data. A plurality of white signals is generated by a minimum value calculating section by using the minimum luminance value and the minimum luminance value and any one of a plurality of white signals is selected by a white color selecting section according to a selecting signal. A multiplying section multiplies white data by each constant of a weighing factor of each of the R, G, and B colors, resulting in the production of compensation white data and also multiplies the generated compensation white data by the three color corrected data, thus resulting in the production of primary three color data. A dividing section divides the primary three color data by the maximum luminance value, thus resulting in the production of secondary three color data and the color correcting section generates the primary four color data by using the compensation white data, three color corrected data, and secondary three color data. The gamma converting section makes a gamma correction to the primary four color data to finally generate four color data and supplies the four color data to the timing controller.
Moreover, in a liquid crystal display device described in Patent Reference 3 [Japanese Patent Application Laid-open No. 2007-041595 (Abstract, FIGS. 1 and 2)], a given RGB video signal is converted into an RGBW video signal via a signal converting section and rendering processing is performed thereon. After that, the processed RGBW video signal is stored, on a temporary basis, in a buffer section and also the RGBW video signal fed from the buffer section is supplied to a liquid crystal panel.
However, the devices described above have the following problems. That is, in the display device disclosed in the Patent Reference 1, when all of the three color of RGB are inputted with gray levels, if the minimum luminance value M2 becomes 0.5 or less, a gray level value of the color corresponding to the minimum luminance value M2, out of the colors RGB becomes 0 (in the above operation example, gray level value Bout′ is 0), displaying is performed at gray levels R, G, and B (in the above operation example, gray level values Rout′ and Gout′) R and G, except the white color and the minimum luminance value M2. On the other hand, from a time point when the minimum luminance value M2 exceeds 0.5, the gray level value of the white subpixel reaches 255 being an upper limit, displaying is performed by using gray level values of RGB corresponding to the minimum luminance value M2 with the gray level value of the white subpixel being fixed to be 255. Thus, in the display device, with the minimum luminance M2 (=0.5) being used as a border, two kinds of behavior occurs, one being the case where white is displayed by the white subpixels only and another being the case where white is displayed by the RGB subpixels with the gray level being fixed to be W=255.
On the other hand, in the case where the display device is made up of a liquid crystal panel, in many cases, each of the R subpixel, G subpixel, and B subpixel is constructed of a pigment for a color filter, however, a white subpixel is not constructed of the pigment, but of an overcoat material to remove concave and convex portions that may occur between the R, G, and B subpixels and the white subpixel. This causes a difference in spectral characteristics among white pixels, resulting in the phenomenon in which chromaticity coordinates of each white differ in a chromaticity diagram. In this case, for example, as shown in FIG. 14, there is a difference between chromaticity coordinates (Δ) of white produced by three subpixels of R, G, and B and chromaticity coordinates (⋄) of white produced by the white subpixel, thus resulting in the appearance of an inflection point at each line of demarcation among colors in a chromaticity diagram in the RGBW four color colorimetric system. Also, as shown in FIG. 15, in some cases, an inflection point appears in the characteristic of gray level versus luminance occurring when cyan is changed to be white by two color mixing of cyan and red. Thus, a problem occurs that, due to the inflection point in each characteristic, a singular point (for example, gamma characteristic abnormality or a like) appears on a display screen.
In addition, in the liquid crystal display device described in the Patent Reference 2, three color source data is converted by data converting section into four data, however, its configurations are different from the present invention.
In the liquid crystal display device described in the Patent Reference 3, an RGB video signal is converted into an RGBW video signal by a signal conversion section, however, its configurations are different from the present invention.